i think this is kind of a personal choice situation. if it were me i would either use one or none for primer and at least 2 coats of paint. a darker color i will always prime before painting it a lighter color but a lighter color i rarely prime if i'm going darker. having a grey tint in your primer isn't a bad idea but what shade of grey would depend on what color you choose to paint the room. a very deep dark color would benefit from a darker grey while a light color would be better with a very light grey, almost antique white.